I'm just posting this up to help others as my research into this issue revealed very little. My wife's 2007 G6 GT convertible with 3.9 would slip and then grab upon taking off from a dead stop. I tried changing the fluid and filter which brought an improvement, but no cure. When we shifted in in manual mode, it worked like it should. The pressure control solenoid was the suspected culprit, so I ordered a 5 piece solenoid kit off of Ebay (http://www.ebay.com/itm/230775281062...84.m1423.l2649
) for $82 shipped. I started in on the car Saturday morning and I eventually had all solenoids replaced by 9:00 pm. To make a long story short, you have to remove the battery tray (unhook two fuse boards closest to radiator), undo the transmission mount located under it and remove it, unhook the lower control arm, remove the four bolts for the cradle (have engine supported from above with a beam that spans both fenders. There are "stub" brackets that protrude through the front
bumper cover that hold up the front of the cradle and hook into plastic end tanks on radiator-I'm lucky I didn't break the radiator. I think they are the only things that held the cradle up-be careful), undo pinch bolt on on steering shaft and bolts/nuts holding steering rack, I undid the front engine mount and discovered the rear mount after I was almost finished. I also undid the shift cable and the roll bar on the driver's side. Once all of this is done, the driver's side of the engine has to be lowered enough to remove the bracket off of the side cover. The cradle has to be lowered enough to get a Torx bit onto the four "screws"in the bottom of the side cover. Then the remaining side cover bolts can be removed. I tried every way imaginable to squeeze the side cover out of there, but it wasn't coming out. I finally gave up and scooted it toward the front enough to replace the pressure manifold and one shift solenoid. I then moved it rearward enough to BARELY get to the pressure control solenoid, TCC solenoid and the other shift solenoid. All four solenoids are held in with a "clothes pin" that would be child's play if it was not behind the edge of the cover and the wiring harness(needle nosed vise-grips are worth millions). I also replaced the output shaft seal while I was in there; be aware that there is an O-ring that seals that big hole in the cover. I put the cover back on and called it a night. It took me another 4-5 hours to reassemble everything. I also pulled the bottom pan and wiped out the metal dust and put in 2 GM magnets (I saw there was a TSB on this) in the pan; the pressure control solenoid is said to act like an electrical magnet (the TCC solenoid and pressure control solenoid have VERY fines mesh screens for the fluid to pass through that may get clogged up) Did I mention I have a lift in my garage?
Observations- this is not a job for the novice or intermediate week-end mechanic. If you don't have the proper equipment to safely move and support the engine, you're probably going to be real frustrated and get hurt.
If you're removing that cover, replace all that I mentioned(you don't want to do it again). If you have the engine out of the car or can get to the side cover, you might want to replace all of these items prior to putting the engine into the vehicle-it's probably a 1 job). If you haven't changed the fluid and filter every 50,000 miles, I'd start real soon( I changed it the first time at 70,000 miles and 2 days ago at 81,000). Prior to pulling the side pan, I'd try to fix the grabbing issue by changing the fluid/filter (re-use original pan gasket) and installing at least one more of the GM magnets ( AC Delco 29535617 Transmission Magnet ). Walmart Syntech (it meets GM specs) fluid is $4.27/qt vs about $8.00 at O'Reilley's. If that doesn't work, have fun. Oh, the transmission shifted beautifully today!